Newton's third law to explain lift

  1. Ok so I just read this statement as part of an explaination of list using Newton's 3rd law

    "The amazing thing about wings is that because they are flying through air which is a fluid, the top of the wing deflects air down as well as the bottom of the wing."

    What I don't understand is how the top of the wing deflects air downwards. Anyone care to explain?
     
  2. jcsd
  3. I have no idea what that statement said.

    Wings fly because their shape creates overpressure under and underpressure over them.
    That's why racing cars have "inverted" wings, to keep them on the ground

    http://wings.avkids.com/Book/Animals/Images/wing_diagram.gif
     
  4. What you just said is an explanation of lift using Bernoulli's principle. Another explaination is using Newton's 3rd law which is what I'm talking about.
     
  5. rcgldr

    rcgldr 7,517
    Homework Helper

    This web site does a descent job of explaining it, with a lot of emphasis on Coanda effect, but towards the end of this web site, there's a diagram of a wind blowing over a roof, and although the air downwind of the roof is turbulent, it's also at lower pressure, due to what some call "void" effect: when a solid object passes through a fluid, or when a fluid passes around a solid object, low pressure "voids" are created because the solid object blocks or diverts the fluid flow away from these low pressure areas.

    After visiting a large number of web sites, my conclusion is that lift is a combination of Coanda and "void" effects.

    http://user.uni-frankfurt.de/~weltner/Mis6/mis6.html
     
    Last edited: Apr 20, 2008
  6. D H

    Staff: Mentor

    That's a misapplication of Bernoulli's principle to explain lift. This misapplication is unfortunately used quite widely. The attached diagram is simply incorrect. If the plane is to go up (or even stay level), something else must go down. That something else is air: The wings deflect air downward. Lift results from Newton's third law.

    How the bottom side of the wing accomplishes this downward deflection is obvious. How the top of the wing contributes to this deflection at all is a lot less obvious. Moreover, in a good wing, it is the top rather than the bottom that does the bulk of the work in deflecting air downward. The link Jeff Reid posted does a nice job of explaining the Coanda effect.
     
  7. russ_watters

    Staff: Mentor

    I would say that the sentence you quoted from Crazy Tosser is correct. The application of that sentence (the diagram he linked) is incorrect. The error is that the diagram shows the air straightening out after passing the wing instead of continuing downward as Newton's third law demands.

    Also, just a little note - it is quite common for the lower surface of a wing to contribute nothing at all to the lift being generated by that wing.
     
  8. rcgldr

    rcgldr 7,517
    Homework Helper

    Actually this would be rare. Even in the case of flat bottom air foils which are less efficient, but easier to manufacture, than fully cambered airfoils, the aircraft that use them need a signficant angle of attack to fly. I recall an article mentioning that a Cessna would need a speed around 300mph instead of 150mph to fly with the flat bottom airfoil lower surface horizontal, instead of pitched upwards. Most commercial aircraft fly with a significant angle of attack, especially if they are near capacity, getting some amount of lift from the fuselage itself. The pitched up attitude is enough that you can feel it in the seats, and it's clear that the snack or meal carts need their brakes in order to keep from rolling to the back of the aircraft.

    For normal air foils, most, but not all, of the lift force comes from above the wing, with a few exceptions like this lifting body prototype:

    M2-F2 glider with F104 chase plane:
    m2-f2.jpg

    M2-F3 rocket powered model (reached a speed of Mach 1.6) with B52:
    m2-f3.jpg
     
  9. First of all, aircraft wings are usually fixed to the body at a slight positive angle. This is referred to as the wing's angle of attack. If you look at an aircraft sitting flat on the runway, with the cockpit facing left, you should be able to see this. The wing is actually turned clockwise a few degrees; the wing is pitched up, even when the aircraft body is flat. They do this because typical airfoil/wing geometries generate lift only when the wing sits at a positive angle of attack. Thus, by affixing the wing to the aircraft in this way, the aircraft will generate lift even when the body is sitting flat on the runway.

    Now, imagine the aircraft on the runway again, with the cockpit facing to the left, as before. As the aircraft takes off, the airflow hits the wing, and "attaches" to it as it flows around it, assuming that the wing is properly designed and the flow remains laminar. Laminar flow basically just means that the airflow goes smoothly and continuously around the wing. From this, and considering how we just described the positive pitch of the wing, it's easy to see that the region of flow approaching the wing from the front turns downwards as it follows the shape of the wing. Hence, the top of the wing deflects the air downwards, but it's not like it's the top of the wing acting alone; it's really the entirety of the wing shape and geometric composition.
     
    Last edited: Apr 22, 2008
  10. russ_watters

    Staff: Mentor

    I'm not so sure. Here's the data for a 172: http://www.prism.gatech.edu/~gtg635r/Lift-Drag Ratio Optimization of Cessna 172.html

    Here's a picture of the airfoil: http://www.windmission.dk/workshop/BasicBladeDesign/naca.html

    It's not flat on the bottom, it's curved, which means that at zero AoA the bottom surface has a negative contribution to the lift. By my calculation, with a .25 CL at 0 AoA, and a cruise weight of 2000 lb, it would need to be flying at 133 mph. Cruise speed for the 172 is 130mph. This is likely by design, as the minimum Cd tends to occur near 0 AoA.

    Also: this link appears to show that at 6 degrees AoA, the bottom surface produces roughly zero lift (due to the velocity averaging 1): http://www.mh-aerotools.de/airfoils/velocitydistributions.htm
     
    Last edited: Apr 22, 2008
  11. rcgldr

    rcgldr 7,517
    Homework Helper

    I get:
    ((.25-.053)/4.56)*(180/pi) = 2.475 degrees AoA.

    There's signifcant area below the 1. I updated the picture with a line going across v=1:

    velo6.gif
     
  12. That data is a bit off. A 172 cruises at 105KTS ~120mph, NOT 120KTS.

    At 120Kts, your 7kts away from the maximum structural crusing speed. Use a max weight of 2550lbs for your calculation.
     
    Last edited: Apr 22, 2008
  13. Careful, thats the angle of incidence. Not attack.
     
  14. rcgldr

    rcgldr 7,517
    Homework Helper

    Since I didn't edit the post in time, I'm reposting:

    Using the formula from that link I get:
    ((.25-.053)/4.56)*(180/pi) = 2.475 degrees AoA.

    A NACA 2412 wing with no washout would need less AoA to produce the same CL. The article also states the washout backwards, as it's the tips that get a negative AoA relative to the root. I don't know what the root incidence is.

    There's signifcant area below the 1. I updated the picture with a line going across v=1:

    velo6.gif

    At 2.475 degrees, there would be very little lift generated below the wing. I'm not sure how much increase in AoA would be required to cruise at a more reasonable (than sea level) 5000 feet.

    Commercial airliners cruise at over 30,000 feet, and at a pretty significant AoA, although I haven't been able to find exact numbers.
     
  15. russ_watters

    Staff: Mentor

    What is that calculation? I'm using the lift equation to show how much lift you get at 0 AoA: Cl = Lift / ((1/2) * (rho) * ((velocity) ^ 2) * s)

    .25=2000/(.5*rho*v^2*s)
    .25=2000/(.5*.002377*v^2*175)
    v^2= 196 fps = 134 mph

    (I used a crusie weight of 2000lb, though Cyrus is saying I should use higher)
    You're right, I was thinking upside-down with that one.
     
  16. Last edited: Apr 22, 2008
  17. Thats not going to be very accurate though. You are ignoring the fact that the fuselauge is creating lift, and that the tail is creating negative lift.
     
  18. rcgldr

    rcgldr 7,517
    Homework Helper

    I don't know what the incidence is, only that the article states that there is 3 degrees of wash-out, and the conflicted itself by stating incidence was -3 at the root and 0 at the tips, which would be wash-in, so this is wrong.
     
  19. russ_watters

    Staff: Mentor

    Here's a link that discusses the issue and lists a plane with a 115kt cruise speed and a 4.5 degree AoA: http://www.av8n.com/how/htm/aoa.html

    Note: this link uses the zero-lift definition of AoA, not the geometric definition. Using the previous link, the zero lift point is about -2 deg AoA for that wing, making the geometric AoA for the above link about 2.5 deg at cruise. Given cyrus's clarification of the performance, I'm willing to accept 2.5 deg instead of 0 for cruise.

    Unfortunately all we've established is that somewhere above 0 deg and below 6 deg, the lower surface's contribution is 0, but some planes cruise with AoA's to the lower end of that.

    In any case, we're getting a little specific about our scenario. The point that led to this little side argument just said that it was "common". I didn't specify cruise or max gross weight or anything. The point is that there are a lot of scenarios where a signficant amount of lift can be generated by the wing with no contribution by the lower surface.
     
  20. russ_watters

    Staff: Mentor

    We're getting too bogged down in specifics here anyway. All I was trying to accomplish with the last sentence in post 6 was to point out that there are lot of scenarios where the lower surface of a wing could contribute nothing to the lift generated by the wing.
     
  21. incidence is the angle the wing makes with the fuselage.
     
Know someone interested in this topic? Share a link to this question via email, Google+, Twitter, or Facebook

Have something to add?